This invention relates to a speed sensor for a rotating body in which a pick-up coil senses the passing of one of a plurality of teeth of magnetic material attached around the rotating body and the speed is measured from pulses converted from the output signal from the pick-up coil.
An electro-magnetic pick-up coil is generally superior to an optical sensor in its durability and maintainability. That is why a pick-up coil is frequently adopted in rotational speed sensors for use in various controllers requiring rotational speed as one of its control parameters. An anti-skid controller for a vehicle is disclosed, for example, in Published Unexamined Japanese Patent Application No. 60-25836, corresponding to U.S. Pat. No. 4,670,852, in which a speed sensor utilizing a pick-up coil is included. Here the rotational speed is measured from the cycle time of pulses and the cycle time is counted based on an arbitrarily predetermined one of the rising edge or falling edge of the pulses.
It is known that the output signal of the pick-up coil is ideal when the teeth and the spaces between the teeth are regularly arranged around the rotating body (preferably the ratio of [width of a tooth]:[width of a space]is 1:2) and the core diameter of the pick-up coil is set equal to the width of a tooth.
Further a method is already known (for example, in the document described above) to obtain accurate rotational speed by compensating for any loss of an output pulse due to a loss of a tooth and by eliminating sporadic error signals such as externally generated noise pulses.
In some cases, accuracy of the speed measurement is still deteriorated by other factors as set forth below.
One factor is the miniaturization of the rotor having the teeth and spaces. As described above, the tooth width, the space width and the core diameter should have a predetermined ratio in order to obtain an ideal output signal. Under such condition, when the size of the pick-up coil is determined, reduction in size of the rotor results in reduction in the number of teeth and spaces around the rotor. This leads to a longer interval between pulses (which is generated by a comparator from the output signal of the pick-up coil) at the same rotational speed. For example, the number of teeth around the rotor is halved when the rotor diameter is halved and the cycle time of the pulses is doubled at the same speed. Therefore, at a low speed, it may occur that the aforementioned predetermined edge (rising edge or falling edge) is not detected during a preset pulse monitoring period. In this case, speed measurement is impossible. Even when the predetermined edge (say, falling edge) is detected in the pulse monitoring period, it may sometimes occur that the pulse monitoring period and a speed calculation period for measuring the pulse cycle are out of phase. This leads to a slower response of the speed calculation to the actual speed change. In summary, the miniaturization of the rotor may lead to miscalculation or delayed calculation of the rotational speed.
Another factor is the eccentricity of the rotor (an array of teeth) with respect to the center of the rotating body. This factor influences the accuracy of speed measurement independent of the size of the rotor. The mechanism is explained with reference to FIGS. 7A-7G. When the teeth and spaces of the rotor (FIG. 7A) pass the core of the pick-up coil (FIG. 7B), an electrical signal output from the pick-up coil is shaped as shown in FIG. 7C. As shown in FIG. 7C, the peak of the output signal is obtained when a tooth leaves the core (at B and E) and when a tooth passes the core (at C). When the center of the core coincides with the center of a tooth (at A and D) or a space, on the other hand, the output signal is zero. As the width of a tooth and the width of a space are not the same, the length of the region in which the output signal increases (such as from C to E) is not the same as the length of the region in which the output signal decreases (such as from B to C). Namely, the gradient of the output signal is different between those regions. Whether the increasing side slope is steeper or the decreasing side slope is steeper depends on the winding direction of the wire of the pick-u coil. The solid line and the broken line of FIG. 7C show the two cases where the winding directions are opposite.
When an eccentricity of the rotor exists, the base of the output signal makes a low frequency wave whose cycle time is the same as the cycle time of the rotation of the rotor, as shown in FIG. 7D. Further, the eccentricity makes an amplitude wave superposed on the output signal because the distance between the teeth and pick-up coil varies during one rotational cycle of the rotor. The resultant shape of the output signal from the pick-up coil when an eccentricity exists is shown by a solid line in FIG. 7F. The two-dot broken line in FIG. 7F shows the normal output signal without the eccentricity. When the output signals are converted into pulses by a comparator, as shown in FIG. 7G, the positions of the rising edge and the falling edge of the pulses are influenced by the eccentricity. In other words, the position of the rising edge (which corresponds to the less steeper side slope of the output signal) is shifted out of normal position more than is the falling edge. As shown in FIG. 7G, the deviation of the rising edge (.DELTA.TE2) from the normal position is generally greater than that of the falling edge (.DELTA.TE1). Thus, when the rising edge is selected for the measurement of the cycle time of the pulse signals, the accuracy of the rotational speed measurement deteriorates. The accuracy deteriorates more as the speed of the rotating body increases because more pulses having the deviation are counted during a preset monitoring period. In practice, it is very difficult to avoid the effects of eccentricity.